1) Field of the Invention
The present invention relates to a thin film transistor that suppresses aging of a threshold voltage.
2) Description of the Related Art
Image display apparatus with the organic light emitting diodes (LEDs) are attracting great attention. The reason being that the organic LEDs have self-luminescence, and therefore, the organic LEDs do not require a back light. The organic LEDs have better response, contrast, and visibility as compared to the liquid crystal devices. Since the structure of the organic LEDs is simple, the organic LEDs can be manufactured at low cost.
FIGS. 9A and 9B illustrate a conventional image display apparatus. As shown in FIG. 9A, a gate electrode of a thin film transistor 102, which functions as a driver, is connected to an anode electrode of an organic LED 101. A potential applied to the gate electrode of the thin film transistor 102 is adjusted to control an electric current that flows through the organic LED 101. The luminance of the light emitted from the organic LED 101 varies in accordance with the current flowing through it. Thus, the luminance of the organic LED 101 can be readily controlled by controlling the gate potential of the thin film transistor 102.
A not shown second thin film transistor controls the gate potential of the thin film transistor 102. The second thin film transistor functions as a switching device. The second thin film transistor receives a display signal and a scan signal from a predetermined data line and a predetermined scan line respectively, and controls the gate electrode of the thin film transistor 102. The self-luminescent image display apparatus is realized by providing an array of the structure shown in FIG. 9A and the second thin film transistor, corresponding to the number of pixels.
FIG. 9B illustrates a top view of the thin film transistor 102. The thin film transistor 102 has a channel 104 sandwiched between source/drain electrodes 103 and 105. An electric current is induced in the channel 104 when a predetermined potential is applied to the channel 104 from the gate electrode of the thin film transistor 102. The channel 104 has a semiconductor layer that has the same conductivity as that of a source/drain region that is disposed below the source/drain electrodes 103 and 105. As a result, an electric current can flow from the source/drain electrode 103 to the source/drain electrode 105 and vice versa.
However, the conventional image display device has many problems. Aged degradation of the thin film transistor that controls the luminance of the organic LED is one of the problems.
When displaying an image, the organic LED corresponding to each pixel is turned on. In other words, the thin film transistor 102 is always kept on, and an electric current of a necessary level is made to flow through the channel 104 to make the organic LED emit a light of desired luminance.
It is known that the density of the current in the channel 104 is not uniform. For example, the current gets concentrated near the ends of the channel 104 due to the electric field integration at the ends. When the switching device of the liquid-crystal display apparatus functions as a voltage driving device, the anisotropy of the current density is not a problem. However, in the image display apparatus, a current flows through the channel 104 over a long time. Therefore, the thin film transistor 102 is physically damaged due to the current concentration, which causes a variation in the threshold voltage.
When the threshold voltage changes, the current that flows through the channel 104 deviates from a desired level, even when the same potential is applied to the gate electrode of the thin film transistor 102 that controls the light emission state of the organic LED 101. Corresponding to the variation in the current that flows through the channel 104, the current that flows through the organic LED 101 also changes. Therefore, the change in the threshold voltage of the thin film transistor 102 directly affects the luminance of the organic LED 101. As a result, the image definition is degraded. Specifically, when the threshold voltage of the thin film transistor 102 deviates, a sticking of the screen occurs and the luminance changes, which shortens the life of the image display apparatus.